Property modulation of ternary copolymer via the diverse arrangements of two different repeating units for polymer solar cells and thin film transistors

Abstract

Four ternary copolymers (RP1−4) containing electron-rich 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b’]dithiophene (BDT) and electron-deficient 4,6-bis(thiophen-2-yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3-dione (DPPD) and 5-octylthieno [3,4-c]pyrrole-4,6-dione (TPD) units at a 2:1:1 ratio were prepared using four differently modified synthetic procedures to understand the effects of chain growth of the repeating units on the properties of the resulting ternary copolymer. Each copolymer showed a distinct absorption band between 350–700 nm with an optical band gap of 1.91 eV for RP1 and 1.85 eV for RP2−4. On the other hand, the highest occupied molecular orbital (HOMO) energy levels of all four copolymers were identical and the estimated HOMO level was −5.37 eV. All four copolymers, RP1−4, exhibited a hole mobility in the order of 10−4 cm2 V−1 s−1. The polymer solar cells (PSCs) prepared using RP1−4 and PC70BM offered a maximum photovoltaic performances of 5.84%, 5.23%, 5.08%, and 4.86%, respectively. The open-circuit voltage (Voc) and fill factor (FF) of the PSC devices made from RP1−4 were similar but the short-circuit current (Jsc) was different, which affected the photovoltaic performance. These results show that the properties of ternary copolymers are affected significantly by the diverse arrangements of two different repeating units of the ternary copolymer.